1. Field of the Invention
The present invention relates to a plasma display panel, and more particularly, to a method and apparatus for driving a plasma display panel in which false contour can be reduced.
2. Description of the Related Art
As the information processing system advances and its supply is expanded, a display device is of increasing importance as visual information transmitting means. A cathode ray tube forming a main stream of such display devices has disadvantages of a large volume, a high operation voltage and a screen distortion. Recently, a Liquid Crystal Display (LCD), a Field Emission Display (FED) and a Plasma Display Panel (PDP) are under development for solving the aforementioned disadvantages of the cathode ray tube.
Among such flat display devices, the PDP is a device to display a picture through excitation and radiation of a phosphor by a vacuum ultraviolet generated at the time of discharging an inert mixture gas. The PDP has advantages in that it can be slimmed and large-sized, its manufacture is easy due to a simple structure, and luminance and radiation efficiency are higher than those in other flat display devices. Especially, an alternate current surface discharge PDP has advantages of a low voltage operation and a long life since a wall charge is accumulated on a surface at the time of a discharge and the accumulated chargers protects the electrodes from sputtering generated by the discharge.
FIG. 1 illustrates a conventional three-electrode alternate current surface discharge type plasma display panel.
Referring to FIG. 1, the alternate current surface discharge type PDP includes a front glass substrate 1 having a front electrode 9 formed thereon, and a rear glass substrate 2 having an address electrode 4 formed thereon. The front glass substrate 1 and the rear glass substrate 2 have a barrier rib 3 interposed therebetween and are distant away from each other in parallel. A mixture gas such as Ne+Xe, He+Xe, He+Ne+Xe is injected into a discharge space provided by the front glass substrate 1, the rear glass substrate 2 and the barrier rib 3.
Two front electrodes 9 are paired within one plasma discharge cell. Each of the front electrodes 9 includes a transparent electrode having a wide width, and a bus electrode having a narrow width connected to an edge of one side of the transparent electrode. Any one of the paired front electrodes 9 is used as a scan electrode for allowing a facing discharge to be generated together with an address electrode in response to a scan pulse supplied at an addressing period, and then allowing a surface discharge to occur with an adjacent front electrode in response to a sustain pulse supplied at a sustain period, and the other one is used as a sustain electrode paired with the scan electrode to commonly supply the same sustain pulse therethrough.
A front dielectric layer 7 and a protective layer 8 are layered on the front glass substrate 1 having the front electrode 9 formed thereon. When a plasma discharge is performed, the front dielectric layer 7 limits a discharge current and concurrently, charges the wall charge therein. The protective layer 8 is formed of magnesium oxide (MgO). The protective layer 8 prevents the front dielectric layer 7 from being damaged by the sputtering generated at the time of the plasma discharge, and increases an emission efficiency of a secondary electron.
A rear dielectric layer 6 is formed on the rear glass substrate 2 to cover the address electrode 4. The rear dielectric layer 6 protects the address electrode 4. The barrier rib 3 for partitioning the discharge space is formed on the rear dielectric layer 6. On surfaces of the rear dielectric layer 6 and the barrier rib 3 is coated a phosphor 5 excited by the vacuum ultraviolet for generating visible rays of Red (R), Green (G) and Blue (B).
Generally, in order to express a gray scale of a picture, the PDP is time-division driven in a so-called Address and Display Separated (ADS) way of performing a separation into an address period during which a pixel is selected, and a sustain period during which a display discharge is generated from the selected pixel. That is, one frame period is divided into several sub-fields having the number of the sustain pulses (that is, sustain discharge times) that are differently set according to a luminance weighting value. Each of the sub-fields is divided into a reset period, the address period and the sustain period. For example, in case that it is intended to express a gray scale of 256, a frame period (16.67 ms) corresponding to 1/60 second is divided into eight sub-fields (SF1 to SF8) as shown in FIG. 2. Additionally, each of the eight sub-fields is divided into the reset period, the address period and the sustain period as described beforehand. At this time, the reset period and the address period are identical with each other every sub-field, whileas the sustain period and the number of the sustain pulses allocated at the sustain period are increased in a ratio of 2n (n=0,1,2,3,4,5,6,7) at each of the sub-fields.
Accordingly, the number of the sustain pulses allocated to each of the sub-fields is combined to express a certain gray scale. For example, in order to express a gray scale of 64, the discharge should be made as many as the number of the sustain pulses that are generated by switching on the sub-fields SF1, SF2, SF3, SF4, SF5, SF6 to respectively accumulate the luminance weighting values of 20, 21, 22, 23, 24, 25.
However, if a moving picture is expressed in the above ADS driving way, contours unpleasant to the eye appear around a moving object thereby deteriorating a screen quality. This is called a false contour. The false contour is caused by a difference between luminous centers on a time axis. Herein, the luminous centers represent temporal light centers of the sub-fields that are switched-on (that is, selected during the address period) within one frame. For example, as shown in FIG. 2, the sub-fields SF1, SF2, SF3, SF4, SF5 are switched on to accumulate the luminance weighting values of 20, 21, 22, 23, 24 so as to express a gray scale of 31, whileas only the sub-field SF6 is switched on to embody the gray scale of 31 only using the luminance weighting value 25 so as to express a gray scale of 32. At this time, the discharge should be made during a long period of as much as the sub-fields SF1, SF2, SF3, SF4, SF5 so as to express the gray scale of 31, but the discharge should be made during a short period of as much as the sub-field SF5 so as to express the gray scale of 32. That is, the gray scale of 31 and the gray scale of 32 have a difference of one gray scale therebetween, but the luminous centers of the gray scale of 31 and the gray scale of 32 are considerably different from each other. That is, as shown in FIG. 2, the luminous center at the time of expressing the gray scale of 31 is positioned after a middle portion of one frame, whileas the luminous center at the time of expressing the gray scale of 32 is positioned at an initial portion of one frame such that each of the luminous centers of the gray scale of 31 and the gray scale of 32 is positioned with a considerable difference in time.
After all, the false contour occurs when the luminous center between adjacent gray scales is rapidly changed on the time axis of the frame when the moving picture is embodied.
For example, as shown in FIG. 3, if a gray scale of 127 and a gray scale of 128 move to the right, when an observer traces an object moving along a locus of (A), he/she perceives a brightness with the gray scale of 127, and when tracing an object moving along a locus of (C), he/she perceives a brightness with the gray scale of 128.
However, if the observer traces the object moving along a locus (B) positioned at a boundary of the locus (A) and the locus (C), he/she perceives a very shining brightness with the gray scale of 255 obtained by accumulating the gray scale of 127 and the gray scale of 128.
This applicant suggests a method for not only enabling a high speed drive, but also improving a contrast and the false contour of the moving picture by using a selective write sub-field at which an on-cell is selected and a selective erase sub-field at which an off-cell is selected during one frame period through a Selective Write Selective Erase sub-field (SWSE) driving way, in earlier filed Korean Patent Application Nos.: 1020000012669, 1020000053214, 1020010003003 and 1020010006492.
However, even the above earlier filed applications have a disadvantage in that the false contour appears since the luminous centers can be differentiated on the time axis when the adjacent gray scales are expressed.